Such a circuit arrangement is known (German Auslegeschrift No. 2,649,024). Here a subcriber's line wire lead with its assigned power source is connected by way of the series circuit of a transmitting diode, serving for loop current detection, of an optical coupler and of a resistor, so that this series circuit and the impedance of the subscriber's line form two branches, connected in series, of a bridge circuit; two further branches are formed by the connection in series of a resistor and a capacitor or a resistor, which are connected between the other subscriber's line wire lead and the junction point of the series circuit consisting of a transmitter diode and resistor with the assigned power source, and in the neutral arm of this bridge circuit is interposed the transmitting diode of another optical coupler, the receiving diode of which, by way of a further interposed amplifier, acts on the first input of the amplifier which emits the outgoing signals. For this the transmitting diode serving for current detection and the resistor which is in series with it in a bridge arm create an asymmetry in the subscriber's line which must be eliminated with a relatively great circuit complexity. Besides this, the above-mentioned bridge circuit and the further connection means connected at its output in the transmission channel for the outgoing signals require a great complexity.